Direct positive images can be produced by use of a certain type of silver halide photographic emulsions. An example is a silver halide photographic emulsion comprising fogged gelatino-halide grains, an electron acceptor, a reducing agent, and a compound of metals more electropositive than silver, as described in U.S. Pat. No. 3,501,307. Such a type of emulsion utilizes a spectral sensitization technique in order to increase the sensitivity. Numerous dyes are known as the spectral sensitizers of negative emulsions. When these sensitizers are used in direct positive silver halide emulsion, such dyes exhibits various defects such as flattening of the characteristic curve and often re-reversal. On the other hand, various spectral sensitizers have been proposed for the direct positive silver halide photographic emulsion but those which have satisfactory sensitizing efficiency are limited in number. Although dyes described, for example, in U.S. Pat. Nos. 3,314,796, 3,431,111 and 3,505,070 exhibit an excellent sensitizing efficiency, but they have a regrettable disadvantage of discoloration (color stain) due to the residual dye remained after photographic processings.
The present inventors carried out an extensive study to search a good sensitizing dye which can be used for direct positive emulsions, and have a high sensitizing efficiency without the above defects.